Returns whether $(D c) is a digit in base 8 (0..7).
The $(D octal) facility is intended as an experimental facility to replace _octal literals starting with $(D '0'), which many find confusing. Using $(D octal!177) or $(D octal!"177") instead of $(D 0177) as an _octal literal makes code clearer and the intent more visible. If use of this facility becomes preponderent, a future version of the language may deem old-style _octal literals deprecated.